This invention relates to tennis balls, and, more particularly, to a tennis ball with magnetic properties which permit an instrument to detect whether the ball is inside or outside of a boundary line.
A persistent problem in the game of tennis is making accurate and consistent judgments of whether or not the tennis ball is inside or outside of boundary lines on the tennis court. Tennis tournaments use line judges who attempt to make a visual determination of whether the ball is in or out on the service and during subsequent play. However, any person who is even a casual fan of tennis is familiar with the arguments which commonly occur between players and line judges over the correctness of the judge's call. The problem is exacerbated when a line judge'call is overruled by the chair umpire, who presumably does not have as good a view of the line as the line judge.
Attempts have been made to provide automatic detection of whether a tennis ball lands inside or outside a boundary line. For example, some tennis balls have been provided with a metallic device which is intended to close an electrical circuit between wires which are embedded in the court to provide an audible signal when the ball is out. More recently, attempts have been made to provide a magnetically detectable ball which can be sensed by an instrument which measures the magnetic permeability of the ball while in motion.
One such magnetic detection instrument is produced by a company named Tel Pty. Ltd., from 26-28 Fitzroy Avenue, Camden Park 5038, South Australia. Although the details of the manner in which the instrument operates are not known, it is believed that the instrument measures the magnetic flux or magnetic permeability of a ball which has ferromagnetic permeability incorporated in it. According to published information from Tel, the Tel detection system has four components: antenna arrays buried below the court lines which transmit and receive data, an instrument box holding 13 computers (one for each line), a hand-held computer operated by the chair empire, and tennis balls which contain metal particles embedded in the rubber core. When a moving tennis ball is within about four inches above a line, an electronic signal is produced because the magnetic particles in the ball disturb the magnetic field above the line. The Tel system provides information on ball velocity, approach trajectory angle, elevation and position of the centroid of the ball footprint relative to the outer edge of a court line. This information is used by the 13 computers to make in and out decisions, although during play the system makes only out decisions audibly.
One prior art tennis ball which was used with the Tel instrument used an iron powder obtained from AEM Cores Pty. Ltd., Bedford Street, Billman, South Australia 5013 under the name Telsen. The powder had a specific gravity of 7.65.
Tennis balls which incorporated the Telsen powder did not meet the specifications for use with the Tel instrument and did not meet the specifications of the United States Tennis Association (USTA). The average magnetic reading level met the Tel specification, but the range of the readings was too great (88% of the balls failed to meet the specification). The Tel specifications are a total magnetic permeability of greater than 0.6 with a variance (variation in the uniformity of distribution of the magnetic permeability) less than 0.60 as measured by the Tel instrument. The balls did not meet USTA specifications because their deflection was too soft.
USTA specifications for a tennis ball provide that the ball shall have a uniform outer surface, be white or yellow in color, have a diameter of more than 21/2 inches (6.35) and less than 25/8 inches (6.67 cm), and have a weight more than 2 ounces (56.7 grams) and less than 21/16 ounces (58.5 grams). The ball shall have a bound of more than 53 inches (135 cm) and less than 58 inches (147 cm) when dropped 100 inches (254 cm) upon a concrete base. The ball shall have a forward deformation of more than 0.220 inch (0.56 cm) and less than 0.290 inch (0.74 cm) and a return deformation of more than 0.350 inch (0.89 cm) and less than 0.425 inch (1.08 cm) at 18 lb. (8.165 kg) load. The deformation figures shall be averages of three individual readings along three axes of the ball and no two individual readings shall differ by more than 0.030 of an inch (0.08 cm) in each case.